Universal host for RG or RGB emission in organic light emitting devices

ABSTRACT

The present invention describes the use of red, green and, if necessary, blue dopants dispersed in a universal host material as the active emitting layer in OLEDs. The universal host is transparent in the visible region, and may be emissive in the blue region when used as the blue emitting species or possesses carrier transport properties. By dispersing the dopants in the universal host, efficient energy transfer from host to guest and/or direct carrier recombination on the dopant takes place resulting in bright red, green or blue emission, depending on the dopant. The resulting spectra are characteristic of the guest molecules.

RELATED APPLICATIONS

[0001] This is a Non-provisional Application relating to a previouslyfiled Provisional Application, PTO No. 60/253,717, filed on Nov. 29,2000.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to colored light emission in lightemitting device structures for use in a variety of apparatuses.

[0004] 2. Description of the Background Art

[0005] Organic light emitting devices (OLEDs) are an emerging technologythat may soon replace liquid crystal displays (LCDs) in flat paneldisplay applications due to their desirable characteristics includingself-emissive high brightness, wide viewing angles, light-weight, andlow power consumption. Recently, Sony previewed a prototype of anOLED-based display that is slightly thicker than a credit card andannounced production to start in 2003. A display is made up of many tinyindividual pixels (picture elements) where, an OLED represents onepixel. In a full-color display, each pixel contains one or all of thethree color components, namely, red, green and blue (RGB).

[0006] An OLED consists of a transparent substrate, typically glass orplastic, coated with a transparent conducting material, such as IndiumTin oxide (ITO), one or more hole injecting and/or hole transportinglayers (HTL), one or more electron transporting (ETL) and/or electroninjecting layers and a cathode made up of low work function metals. TheHTL or ETL may also have light emissive properties or a separateemitting layer may be sandwiched between the HTL and ETL.

[0007] Developing efficient and economical methods to manufacture RGBpatterned pixels is one of the main issues concerning the realization offull-color flat panel displays. Several approaches have been developedto achieve full-color organic emissive displays. The first methodconsists of filtering white light with RGB band-pass filters. Thistechnique results in a large reduction of the optical power from thewhite OLED. Thus the color-filtered OLEDs must be operated at highbrightness/current density, which may accelerate degradation and shortenthe lifetime of the device. Another method utilizes the conversion ofblue light to green light and red light through a color converting layercomprising a fluorescent material and has been demonstrated with manyvariations (See U.S. Pat. Nos. 5,126,214; 5,294,870; 6,019,654;6,023,371; 6,137,221; 6,249,372, all incorporated by reference herein).A major challenge of this method is the difficulty of finding a redfluorescent material with a high absorption coefficient in the bluewavelength region and having a high fluorescence in the red wavelengthregion. This method also results in reduced device efficiency during thecolor conversion process.

[0008] Yet another method used to achieve RGB emission is through thepatterning of discrete RGB sub-pixels. This method has been demonstratedwith the use of precise shadow masks (See U.S. Pat. No. 6,214,631,herein incorporated by reference). This patterning method has also beenaccomplished with a laser ablation technique (See U.S. Pat. No.6,146,715, herein incorporated by reference) which is used to etch awayundesired organic and electrode layers as a way to avoid using harshphotoresist chemicals to pattern discrete RGB pixels adjacent to eachother on the same substrate. This approach is more advantageous than theothers because the red, green, and blue OLEDs are individually optimizedto achieve high device efficiencies at low power. Typically, threedifferent OLED structures are used in order to optimize each colorpixel, with a minimum of two different materials (host and dopant) foreach of the primary colors. The use of several different types ofmaterial components during device fabrication may increase the risk forcross-contamination and would bring about a more complicated process fordevice fabrication.

[0009] Organic electroluminescent devices that include organic hostmaterials and dopants are disclosed, for example, in the followingpatents and publications, which are all herein incorporated byreference: U.S. Pat. No. 3,172,862 to Gurnee et al; U.S. Pat. No.3,173,050 to Gurnee; U.S. Pat. No. 3,710,167 to Dresner et al; U.S. Pat.No. 4,356,429 to Tang; U.S. Pat. No. 4,769,292 to Tang et al; U.S. Pat.No. 5,059,863; U.S. Pat. No. 5,126,214 to Tokailin et al; U.S. Pat. No.5,382,477 to Saito et al; U.S. Pat. No. 5,409,783 to Tang et al; U.S.Pat. No. 5,554,450 to Shi et al; U.S. Pat. No. 5,635,307 to Takeuchi etal; U.S. Pat. No. 5,674,597 to Fujii et al; U.S. Pat. No. 5,709,959 toAdachi et al; U.S. Pat. No. 5,747,183 to Shi et al; U.S. Pat. No.5,756,224 to Borner et al; U.S. Pat. No. 5,861,219 to Thompson et al;U.S. Pat. No. 5,908,581 to Chen et al; U.S. Pat. No. 5,932,363 to Hu etal; U.S. Pat. No. 5,935,720 to Chen et al; U.S. Pat. No. 5,935,721 toShi et al; U.S. Pat. No. 5,948,941 to Tamano et al; U.S. Pat. No.5,989,737 to Xie et al; International Publication No. WO 98/06242(Forrest et al); C. W. Tang et al “Electroluminescence of Doped OrganicThin Films”, J. Appl. Phys. 65(9), May 1969, pp 3610-3616; C. W. Tangand S. A. VanSlyke, “Organic Electroluminescent Diodes”, Appl. Phys.Lett. 51(12), Sep. 21, 1987, pp. 913-915; C. W. Tang, “OrganicElectroluminescent Materials and Devices” Information Display, October1996, pp. 16-19; J. Shi and C. W. Tang, “Doped OrganicElectroluminescent Devices with Improved Stability”, Appl. Phys. Lett70(13) Mar. 31, 1997, pp. 1665-1667; Shoustikov et al,“Electroluminescence Color Tuning by Dye Doping in OrganicLight-Emitting Diodes”, IEEE Journal of Selected Topics in QuantumElectronics, Vol. 4, No. 1 January/February 1998, pp 3-13; Baldo et al,“Highly Efficient Phosphorescent Emission from OrganicElectroluminescent Devices”, Nature, Vol. 395, Sep. 10, 1998, pp151-153; O'Brien et al “Improved Energy Transfer inElectrophosphorescent Devices”, Applied Physics Letters, Vol. 74, No. 3,Jan. 18, 1999, pp. 442-444.

BRIEF SUMMARY OF THE INVENTION

[0010] RGB emission can be achieved using universal host/dopant systemsas the emitting layer in OLED pixels. This approach allows the displayto be easily color tuned by modifying only one element of the devicestructure, the dopant. The advantage of combining two mechanisms, energytransfer and direct carrier recombination, allows us to use common hostmaterials for different dopants while still maintaining deviceefficiency and good color chromaticity. In addition, this methodminimizes the number of processing steps thus simplifying the devicestructures and reducing the risk of cross contamination. A new featureof this invention is the use of a universal host for RGB dopants toachieve fall color displays using OLEDs. It is the object of thisinvention to provide an approach to simplify OLED structures andminimize the number of materials used to achieve RGB color emission. Afull-color display utilizing a universal host, as described in thisinvention, does not rely solely on good spectral overlap between hostemission and guest absorption (energy transfer) to achieve red emission.The present invention details several possibilities for the usefulnessof this concept. A single universal host can be used for R, G, and Bdopants (see FIG. 1(a)). Additionally, if the universal host has blueemissive properties, it can be used undoped to further reduce the numberof materials. Likewise, if the universal host has carrier transportproperties, the need for additional hole or electron transport materialsis eliminated (see FIGS. 1(b) and 1(c)).

[0011] Another example utilizes the emission properties of either orboth of the carrier transport layers to obtain one or more of the RGBpixels, which again reduces the number of materials used in the devices.A specific example of this last method is described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1(a) shows an OLED structure and its components: a cathode(10); an electron transport layer (ETL) (12); a doped universal host(14); an hole transport later (HTL) (16); and an anode (18).

[0013]FIG. 1(b) shows an OLED structure and its components: a cathode(10); a doped universal host also serving as the ETL (26); an HTL (16),and an anode (18).

[0014]FIG. 1(c) shows an OLED structure and its components: a cathode(10); an ETL (12), a doped universal host also serving as the HTL (28),and an anode (18).

[0015]FIG. 2(a) shows an example of a structure for a red emitting OLEDand its components: a cathode (10); an ETL (12); a hole blocker layer(20); a doped universal host (for emitting red light) (24); an HTL (16);and an anode (18).

[0016]FIG. 2(b) shows an example of structure for a green emitting OLEDand its components: a cathode (10); an ETL (12); a hole blocker layer(20); a doped universal host (for emitting green light) (22); an HTL(16); and an anode (18).

[0017]FIG. 2(c) shows an example of structure for a blue emitting OLEDand its components: a cathode (10); an ETL (12); a hole blocker layer(20); an HTL (16) (material selected so as to emit blue light); and ananode (18).

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0018] The present invention describes a new technique of usingcomposite materials consisting of red, green and blue (if necessary)dopants dispersed in a universal host material as the active emittinglayer in OLEDs. The universal host is a material that is eithertransparent in the visible region or may be emissive in the blue regionwhen used additionally as the blue emitting species and/or possessescarrier transport properties. By dispersing the dopants in the universalhost, efficient energy transfer from host to guest and/or direct carrierrecombination on the dopant takes place resulting in bright red, greenor blue emission, depending on the dopant. The resulting spectra arecharacteristic of the guest molecules.

[0019] The present invention describes a new economical and efficientapproach to achieve RGB emission using a minimum number of materials andOLED structures. In the instant approach, one organic material serves asthe universal host for red, green and blue dopants. This universal hostis a material that is either transparent in the visible region or it maybe emissive in the blue region. Unless the universal host serves thedual role of host and blue emitter, a high photoluminescence quantumefficiency is only required for the dopants of the host and theuniversal host is transparent. The host may also possess carriertransport properties, which could further simplify the OLED structures.

[0020] The universal host can be used as a pure thin film to achieveblue emission. The universal host may also exist in combination with oneof the RGB dopants and used as the emissive layer in OLEDs. When blue orgreen emitters are doped into the universal host, there is efficientenergy transfer from the host material to the dopant and/or carrierrecombination on the dopant resulting in electroluminescencepredominantly from the dopant. When a red emitter is doped into theuniversal host, direct carrier recombination is the predominant emissionmechanism because there is usually poor spectral overlap of the hostemission and the dopant absorbance.

[0021] This universal host has many potential applications inopto-electronic devices such as flat panel electronic displays. Such auniversal host allows OLED displays to be easily color tuned bymodifying the dopant while maximizing the device efficiency.Consequently, the number of materials used and the cost of manufacturingthe displays are greatly minimized.

EXAMPLE 1

[0022] RGB emission was achieved with a universal host used for RG OLEDpixels fabricated with similar device structures, with the exception ofthe emitting species. The blue emissive properties of the hole transportmaterial, 4,4-bis(1-naphthylphenylamino)biphenyl (NPB) were utilized forthe B OLED pixel (see FIG. 1(c)).

[0023] OLED RGB device structures and organic materials used in thisexample are shown in FIGS. 2(a) through 2(c). All materials were vacuumdeposited inside a chamber under a base pressure of approximately 10⁻⁷Torr. All devices contain of a glass substrate coated with a transparentanode material, here indium tin oxide (ITO). In addition, the holetransporting layer in all devices is NPB. The hole blocking layer isbathocuproine (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)(BC) in allthree devices. Lastly, all devices utilized5,5′-bis(dimesitylboryl)-2,2′-bithiophene (BMB-2T) (See Noda, et al.,Adv. Mater., 11, 283 (1999), herein incorporated by reference), as theelectron transport layer.

[0024] For the red and green OLED devices (FIGS. 2(a) and 2(b),respectively), a composite film of the universal host material, BMB-2T,and dopant, was inserted between the NPB and BC layers to act as theemitting layer. 6,13-diphenylpentacene (DPP) andN,N′-diethylquinacridone (DEQ) (See Murata, et al., Proc. SPIE, 3476, 88(1998), herein incorporated by reference), were used as the red andgreen dopants, respectively. Although BMB-2T can be used as a blueemitting material without any dopant, better chromaticity coordinatescan be achieved by using NPB as the blue emitting layer in the bluedevice (see FIG. 2(c)). The BC layer acts as a hole blocker and thusforces recombination inside the NPB layer. A magnesium and silver alloywas used as the cathode for all of the devices.

EXAMPLE 2

[0025] We have also postulated that RGB emission can be readily achievedwhere the universal host used for RG OLED pixels is fabricated withsimilar device structures as described above in Example 1. The blueemissive properties of the universal host can be utilized for the B OLEDpixel. The devices consist of glass substrate coated with indium tinoxide (ITO), a transparent anode material. For all of the devices, thehole transporting layers are NPB. The electron transport layers arecomposed of separate layers of o-TTA and5,5′-bis(dimesitylboryl)-2,2′-bithiophene (BMB-2T). The o-TTA layer alsofunctions to allow for generation and transmittance of blue lightbecause it serves to prevent the formation of an exciplex between theNPB and BMB-2T layers (See Shirota, J MAT Chem., 10, 1-25 (2000), hereinincorporated by reference). For the green and red devices, a compositefilm of the universal host material, BMB-2T, and dopant, would beinserted between the o-TTA and BMB-2T layers to act as the emittinglayer. 6,13-diphenylpentacene (DPP) and N,N′-diethylquinacridone (DEQ)can be used as the red and green dopants, respectively. A magnesium andsilver alloy would be used as the cathode for all of the devices.

[0026] Electroluminescence (EL) spectra were measured inside a glove boxpurged with dry nitrogen. The luminescence was collected and brought outthrough an optical fiber. Voltage-current-luminance measurements wereperformed with a high current source and luminance meter. Deviceperformance was evaluated based on the external quantum efficiencydefined as the ratio of the number of emitted photons to the number ofinjected carriers. Color chromaticity coordinates of theelectroluminescence emission were calculated according to the definitiondeveloped by the Commission Internationale de L'Ećlairage 1931 (CIE1931). Color chromaticity of RGB emission obtained were highlycomparable to those of current cathode ray tube monitors. Direct carrierrecombination on the red guest molecules is likely to be the ELmechanism for the significant improvement of the colore chromaticity andthe EL efficiency of the present invention. The combination of the twomechanisms, energy transfer and direct carrier recombination, allows theinstant invention to utilize the common host materials for differentguests.

[0027] The EL spectra of RGB devices are also very similar tophotoluminescence (PL) spectra characteristic of the pure emittingspecies. The emission from the red device takes advantage of directcarrier recombination on the red dopant molecules (See Adachi, et al., JAppl. Phys., 87, 8049 (2000), herein incorporated by reference). Thisemission mechanism does not rely on spectral overlap of the host anddopant as is necessary for Förster energy transfer. Instead, the dopantacts as a carrier trap in the universal host.

We claim:
 1. An organic light emitting diode (OLED), comprising: auniversal host; a hole transporting layer; an electron transport layer;wherein said hole transporting layer and said electron transport layerare on opposing sides of said universal host, and are in electricalcontact with said universal host; wherein said hole transporting layer,said electron transport layer, and said universal host together comprisean active portion of said OLED; electrodes on opposing sides of saidactive portion for providing a bias across said active portion; whereinat least one of said electrodes is transparent.
 2. The OLED of claim 1,wherein said universal host is a material adapted to emit at wavelengthsin the blue visible light region or shorter.
 3. The OLED of claim 1,wherein said universal host is doped with a red emitting material. 4.The OLED of claim 3, wherein said universal host comprises5,5′-bis(dimesitylboryl)-2,2′-bithiophene, and wherein said red emittingmaterial is 6,13-diphenylpentacene.
 5. The OLED of claim 1, wherein saiduniversal host is doped with a green emitting material.
 6. The OLED ofclaim 5, wherein said universal host material is5,5′-bis(dimesitylboryl)-2,2′-bithiophene, and wherein said greenemitting material is N,N′-diethylquinacridone.
 7. The OLED of claim 1,wherein said universal host is doped with a blue emitting material. 8.The OLED of claim 1, wherein said hole transporting layer is b4,4-bis(1-naphthylpheny-amino)biphenyl.
 9. The OLED of claim 1, whereinsaid electron transport layer is5,5′-bis(dimesitylboryl)-2,2′-bithiophene.
 10. The OLED of claim 1,wherein at least one of said transparent electrodes comprises a glasssubstrate coated with a transparent anode material.
 11. The OLED ofclaim 10, wherein said transparent anode material is indium tin oxide.12. The OLED of claim 1, wherein one of said electrodes comprises ametallic cathode.
 13. The OLED of claim 1, wherein said metallic cathodecomprises an alloy of Mg and Ag.
 14. The OLED of claim 1, wherein a holeblocking layer is inserted between said universal host and said electrontransport layer, and wherein said hole blocking layer, said holetransporting layer, and said electron transport layer are in electricalcontact with said universal host;
 15. The OLED of claim 1, wherein saidhole blocking layer comprises bathocuproine.
 16. An organic lightemitting diode (OLED), comprising: a hole transporting layer; anelectron transport layer that is also a universal host; wherein saidhole transporting layer and said electron transport layer are placed inseries, and are in electrical contact with each other; wherein said holetransporting layer and said electron transport layer together comprisean active portion of said OLED; electrodes on opposing sides of saidactive portion for providing a bias across said active portion; whereinat least one of said electrodes is transparent.
 17. The OLED of claim16, wherein said electron transport layer is a material adapted to emitat wavelengths in the blue visible light region or shorter.
 18. The OLEDof claim 16, wherein said electron transport layer is doped with a redemitting material.
 19. The OLED of claim 18, wherein said red emittingmaterial is 6,13-diphenylpentacene.
 20. The OLED of claim 16, whereinsaid electron transport layer is doped with a green emitting material.21. The OLED of claim 20, wherein said green emitting material isN,N′-diethylquinacridone.
 22. The OLED of claim 16, wherein saidelectron transport layer is doped with a blue emitting material.
 23. TheOLED of claim 16, wherein said hole transporting layer is4,4-bis(1-naphthylphenyl-amino)biphenyl.
 24. The OLED of claim 16,wherein said electron transport layer is5,5′-bis(dimesitylboryl)-2,2′-bithiophene.
 25. The OLED of claim 16,wherein at least one of said transparent electrodes comprises a glasssubstrate coated with a transparent anode material.
 26. The OLED ofclaim 25, wherein said transparent anode material is indium tin oxide.27. The OLED of claim 16, wherein one of said electrodes comprises ametallic cathode.
 28. The OLED of claim 16, wherein said metalliccathode comprises an alloy of Mg and Ag.
 29. An organic light emittingdiode (OLED), comprising: a hole transporting layer that is also auniversal host; an electron transport layer; wherein said holetransporting layer and said electron transport layer are placed inseries, and are in electrical contact with each other; wherein said holetransporting layer and said electron transport layer together comprisean active portion of said OLED; electrodes on opposing sides of saidactive portion for providing a bias across said active portion; whereinat least one of said electrodes is transparent.
 30. The OLED of claim29, wherein said hole transporting layer is a material adapted to emitat wavelengths in the blue visible light region or shorter.
 31. The OLEDof claim 29, wherein said hole transporting layer is doped with a redemitting material.
 32. The OLED of claim 31, wherein said red emittingmaterial is 6,13-diphenylpentacene.
 33. The OLED of claim 29, whereinsaid hole transporting layer is doped with a green emitting material.34. The OLED of claim 33, wherein said green emitting material isN,N′-diethylquinacridone.
 35. The OLED of claim 29, wherein said holetransporting layer is doped with a blue emitting material.
 36. The OLEDof claim 29, wherein said hole transporting layer is4,4-bis(1-naphthylphenyl-amino)biphenyl.
 37. The OLED of claim 29,wherein said electron transport layer is5,5′-bis(dimesitylboryl)-2,2′-bithiophene.
 38. The OLED of claim 29,wherein at least one of said transparent electrodes comprises a glasssubstrate coated with a transparent anode material.
 39. The OLED ofclaim 38, wherein said transparent anode material is indium tin oxide.40. The OLED of claim 29, wherein one of said electrodes comprises ametallic cathode.
 41. The OLED of claim 29, wherein said metalliccathode comprises an alloy of Mg and Ag.